The pics are upside down. The big black letters should run horizontally across the top. They say 宇宙輝星軍旗 (Uchū kisei gunki). The horizontal writing reads from right-to-left. I've never seen this slogan on a flag (of any vintage). "Battle Flag of the Shining Star of the Universe". A bit florid for an army flag, but maybe this was a popular expression in WW2 times. The rest are slogans and names.
平和の戦士 Soldier of Peace in largish letters next to the red disk
忠勇義烈 Fierce dedication to loyalty, bravery, righteousness at the bottom of the disk.
九州男子 Men of
Kyūshū - kind of a slogan of pride, machismo, for men born in the southern island of
Kyūshū. Maybe hinting at the origin of the flag. Left of the disk.
Anyway, its slightly unusual because of the unique slogan at the top. I see no recipient's name on it, which makes it hard to track down any potential surviving relatives. The writing is not particularly good. For me, without a forensic examination of the fibers and pigments on the flag, its impossible to say if its authentic or not. So when evaluating these kinds of flags I personally like to see names and locations, and clear indications of givers and recipients, and a variety of handwriting (showing multiple people signed the flag). Its also nice to see good penmanship, as I'm suspicious of bad, . But that sets the bar fairly high, and not all flags have all of these hallmarks.
So this flag falls outside of my criteria for genuine flags, but the high bar means I probably end up rejecting a few genuine flags here and there. There is a flag posted here on this site just a couple of weeks ago, and it got the thumbs down from me, but it was sent off to a third part for evaluation (
Obon Society) and it looks like it may actually be authentic.